IKK gamma/NEMO Is Required to Confer Antimicrobial Innate Immune Responses in the Yellow Mealworm, Tenebrio Molitor

Abstract

IKK gamma/NEMO is the regulatory subunit of the I kappa B kinase (IKK) complex, which regulates the NF-kappa B signaling pathway. Within the IKK complex, IKK gamma/NEMO is the non-catalytic subunit, whereas IKK alpha and IKK beta are the structurally related catalytic subunits. In this study, TmIKK gamma was screened from the Tenebrio molitor RNA-Seq database and functionally characterized using RNAi screening for its role in regulating T. molitor antimicrobial peptide (AMP) genes after microbial challenges. The TmIKK gamma transcript is 1521 bp that putatively encodes a polypeptide of 506 amino acid residues. TmIKK gamma contains a NF-kappa B essential modulator (NEMO) and a leucine zipper domain of coiled coil region 2 (LZCC2). A phylogenetic analysis confirmed its homology to the red flour beetle, Tribolium castaneum IKK gamma (TcIKK gamma). The expression of TmIKK gamma mRNA showed that it might function in diverse tissues of the insect, with a higher expression in the hemocytes and the fat body of the late-instar larvae. TmIKK gamma mRNA expression was induced by Escherichia coli, Staphylococcus aureus, and Candida albicans challenges in the whole larvae and in tissues such as the hemocytes, gut and fat body. The knockdown of TmIKK gamma mRNA significantly reduced the survival of the larvae after microbial challenges. Furthermore, we investigated the tissue-specific induction patterns of fourteen T. molitor AMP genes in TmIKK gamma mRNA-silenced individuals after microbial challenges. In general, the mRNA expression of TmTenecin1, -2, and -4; TmDefensin1 and -2; TmColeoptericin1 and 2; and TmAttacin1a, 1b, and 2 were found to be downregulated in the hemocytes, gut, and fat body tissues in the TmIKK gamma-silenced individuals after microbial challenges. Under similar conditions, TmRelish (NF-kappa B transcription factor) mRNA was also found to be downregulated. Thus, TmIKK gamma is an important factor in the antimicrobial innate immune response of T. molitor.